This invention generally relates to a novel device, being an analytical apparatus comprising an oscillating piezoelectric sensor and more particularly to an improved quartz crystal microbalance and the use thereof
A quartz crystal microbalance is a device for detecting and measuring very small changes in mass. Its primary components are a quartz crystal and an oscillator circuit coupled to the quartz crystal to produce an output at a resonant frequency of the crystal. The output frequency, which is typically around 10 MHz, is measured to a high degree of accuracy, for example, with a frequency counter. The quartz crystal is, unlike the crystals normally used in electronic circuits, unencapsulated, so that it can interact with its environment. The deposition of small quantities of material onto the crystal changes its resonant frequency and allows the determination of the mass of material deposited. Typically, frequency changes are of the order of a few Hz to a few tens of Hz and changes of the order of nanograms in the mass of material deposited can be detected.
The piezoelectric sensor in research analytical apparatus is typically a thin slice of quartz from an artificially grown crystal, although other piezoelectric materials, such as tourmaline, ethylene diamine tartrate, ADP, KDP, and Rochelle salt can, also be used. Quartz has the advantages of being chemically unreactive and insoluble in water, as well as being relatively temperature insensitive. A typical quartz sensor 10 for a microbalance is shown in FIG. 1. The quartz crystal 16 is typically a circular xe2x80x9cAT-cutxe2x80x9d with metal electrodes 18 on opposing faces. The electrodes are typically sputtered thin (200 nm) films of gold, silver, or titanium, possibly with a sub-layer for improved adhesion. Lead wires 14 attach to the electrodes and also provides mechanical support for the crystal, as well as some degree of isolation from base 12 of the sensor and lead out wires 15. The crystal is typically around 1 cm in diameter. The change in resonant frequency, xcex94F, of an AT-cut quartz crystal of area, A, vibrating in air at fundamental frequency, F, when the mass of the crystal is changed by xcex94M, is given approximately by:
xcex94F=xe2x88x922.3xc3x97106F2xcex94M/A.
The quartz crystal microbalance is most frequently used to measure mass, but can also be used to detect changes in the viscosity and/or density of a liquid, since when vibrating in a liquid all these factors effect the vibrational frequency. Thus, the shift in frequency of a quartz crystal on immersion in a liquid, xcex94F, is given by:
xcex94F=xe2x88x92F03/2(xcex7Lxcfx81Lxcfx81/xcfx80xcexcQxcfx81Q)xc2xd
Where: xcex94F=Change in Frequency
F0=Resonance Frequency
xcex7Lxcfx81L=Liquid absolute density and viscosity
xcexcQxcfx81=Quartz elastic modulus and density.
A quartz crystal microbalance can be used as a bio-sensor, that is as a device which uses as part of the sensor, or is sensitive to, material of biological origin. Typically, part of one or both electrodes of the sensor are coated with material which is capable of binding with a target bio-molecule or cell. When such a receptor is exposed to the target (xe2x80x9cligandxe2x80x9d) compound, the ligand is bound to the substrate causing a change in mass xcex94M of the sensor, and/or viscosity/density changes in the local microenvironment and a consequent change in its vibrational frequency.
There is a general need for improvements in the sensitivity and effectiveness of known quartz crystal microbalances. More particularly, the oscillator drive circuits of conventional microbalances lack stability and precision, particularly when the sensor is immersed in a liquid. When a crystal surface is immersed in a liquid the xe2x80x9cQxe2x80x9d of the crystal (a measure of the sharpness of the resonance or, equivalently, the energy dissipated per cycle) will drop substantially and the resonant frequency will shift slightly. The Q drop is due to the damping effect caused by the absorption of energy by the liquid; the change in resonant frequency is due to the dynamic mass of the liquid on the crystal face. When a crystal resonates its impedance drops from almost infinity to about 50-200xcexa9; when immersed the impedance at resonance can be 100 Kxcexa9 or higher. In order that the apparatus can operate with the sensor both in and out of a liquid it is necessary to provide a system with a wide dynamic range to accurately determine the sensor""s resonant frequency over this wide range of impedances. Although quartz crystal microbalances have been able to operate in either liquid or air, a difficulty arises when the system is required to operate with the sensor in either medium. Furthermore, when operated in a liquid prior devices often exhibit a lack of stability, sensitivity, and precision, possibly related to the broader resonance and greater energy dissipation of the immersed sensor. Efforts to counteract some of this variability have included using dual matched oscillators, comparing the frequencies of working and reference crystals (Dunham G. C., Benson N. H., Petalenz D. and Janata J., Anal. Chem., 67 (1995) 267-272).
There is also a need for improved screening processes for rapid evaluation of compounds of potentially therapeutic benefit. Pharmaceutical companies typically synthesise thousands of compounds, which are then screened in order to identify those which interact with a target molecule. Typically, one or other of the target and ligand are labelled with a radioactive or fluorescent tag, but such techniques are slow, expensive, and require the handling of dangerous materials, and the labelling may interfere with the receptor-ligand interaction. A further problem is therefore the need for improved screening processes and quartz crystal microbalances promise some advantages in this field. However, in this context it is desirable to have a sensor-oscillator system which is stable even in a flowing liquid and which, in this configuration, is sensitive enough to allow bio-molecular interactions on the surface of the crystal electrode to be monitored in real time.
FIG. 2 illustrates an oscillator circuit 20 for a quartz crystal microbalance, known from Barnes C., Sensors and Actuators A., 29 (1991) 59-69. Quartz crystal 24 is series connected in a feedback path connecting the output of CMOS inverter 22 to its input, thereby forming an oscillator. A one-transistor buffer circuit 26 is coupled to the oscillator and provides an output at 28 to a frequency counter. The oscillator will operate in the range 1.7 to 20 MHz, but in the prior art was used by Barnes at 14 MHz. A quartz crystal microbalance using this circuit exhibits the disadvantages outlined above. The present invention seeks to alleviate the problems of conventional systems by providing an improved oscillator circuit.
Accordingly, we will describe analytical apparatus comprising a piezoelectric sensor and an oscillator circuit, coupled to the sensor, to oscillate at a frequency substantially determined by a resonant frequency of the sensor and to provide an output signal at the oscillator frequency at an output and characterised in that the oscillator circuit incorporates means to maintain a substantially constant drive signal to the piezoelectric sensor. In a preferred embodiment, the analytical apparatus is a quartz crystal microbalance. Preferably, the substantially constant drive signal is maintained by AGC means within a feedback loop of the oscillator, the gain control signal from which can be used as an indication of the Q of the piezoelectric sensor. It is desirable that the drive signal to the sensor is substantially sinusoidal since this provides greater accuracy, sensitivity and stability for the apparatus. This can be achieved by ensuring that all the elements in the feedback loop providing signal gain and attenuation are configured to operate in a substantially linear mode.
We will further describe a method of measuring a characteristic of an oscillating piezoelectric sensor in a fluid medium comprising providing an oscillating drive signal to the sensor and characterised in that the method further comprises controlling the drive signal such that it is maintained at an approximately constant level despite changes in the fluid medium.
We will also describe analytical apparatus comprising a piezoelectric sensor; and an oscillator circuit, coupled to the sensor to oscillate at a frequency substantially determined by a resonant frequency of the sensor, and to provide a first output signal at the oscillator frequency; characterised in that the oscillator circuit comprises means to provide a second output signal correlated to the xe2x80x98Qxe2x80x99 of the sensor.
We will further describe an oscillator circuit comprising a piezoelectric sensor, to oscillate at a frequency substantially determined by a resonant frequency of the sensor and to provide an output signal at the oscillator frequency at an output, the circuit comprising an amplifier with feedback coupling to the output to an input of the amplifier to form a feedback loop through the amplifier; and characterised in that the feedback loop incorporates automatic gain control (AGC) means to maintain a substantially constant drive signal to the piezoelectric sensor.